feat(i18n): persist language preference and thread through chat, memoir, TTS

- Add users.language_preference (Alembic 0018, default zh); capture at signup/SMS
  only; expose on auth and profile APIs
- Lite English prompts for chat and memoir; localized stage labels and agent
  names (Life Echo / 岁月知己)
- Tencent TTS: language-aware synthesis, ModelType=1 for 501004, English chunking
- WebSocket pipeline: emit all AGENT_RESPONSE segments when TTS cancels; INFO logs
  for tts_this_turn and TTS decisions; on-demand TTS logging
- Expo: device language on auth, i18n tiers/agent name, [SPLIT] streaming UX fixes
- Tests for migration, prompts, pipeline, router tts_this_turn, reply segments

Co-authored-by: Cursor <cursoragent@cursor.com>

api/app/features/user/service.py

@@ -20,6 +20,12 @@ from app.ports.storage import ObjectStorage
 logger = get_logger(__name__)
 
 
+def _coerce_language(raw) -> str:
+    """Normalize a stored language token to the 'zh' / 'en' Literal."""
+    s = str(raw).strip().lower() if isinstance(raw, str) else ""
+    return s if s in ("zh", "en") else "zh"
+
+
 def _user_to_profile(user: User) -> UserProfileResponse:
     return UserProfileResponse(
         id=user.id,
@@ -33,6 +39,9 @@ def _user_to_profile(user: User) -> UserProfileResponse:
         birth_place=user.birth_place,
         grew_up_place=user.grew_up_place,
         occupation=user.occupation,
+        language_preference=_coerce_language(
+            getattr(user, "language_preference", "zh")
+        ),
     )